This invention relates to photomask blanks, photomasks prepared therefrom and method of manufacturing the same and more particularly a chromium mask, one of hard masks, utilized to prepare such semiconductor elements as integrated circuits and high density integrated circuits.
In recent years, in the manufacture of semiconductor integrated circuits or the like, a hard mask comprising a transparent substrate made of glass, for example, and a metal film formed thereon is used in most cases in view of its durability and high degree of resolution. A typical hardmask is a chromium mask wherein a chromium film is deposited on a glass substrate by vacuum evaporation or sputtering. However, since the reflectance of the chromium film is very high (about 40 to 60%), the resolution of a circuit pattern formed by using the chromium mask is poor due to multiple reflections of the light between a wafer and the chromium mask during exposure.
To obviate this problem a so-called surface reflection free chromium mask has been proposed in which a chromium oxide film having a thickness of 250 to 350 A and acting as a anti-reflection film is formed on the chromium film.
A chromium mask blank provided with a chromium oxide film is generally prepared by the following four methods, but the chromium mask blanks prepared by these methods are not always satisfactry.
A. First method--A chromium oxide film is formed by a vacuum evaporation technique on the surface of a chromium film formed a by vapor deposition technique.
B. Second method--A chromium oxide film is formed by sputtering on the surface of a chromium film formed by sputtering.
C. Third method--A chromium film is formed on the surface of a substrate by vacuum evaporation or sputtering and then the substrate is heated in air or an oxygen atmosphere to convert the surface layer of the chromium film into chromium oxide.
D. Fourth method--A chromium oxide film is formed by vacuum evaporation on the surface of a chromium film formed by sputtering.
However these methods are defective as follows: